This invention relates to light emissive diodes (LED's), and in particular to those of the edge-emissive variety (ELED's). Lasers and superluminescent diodes typically have greater brightnesses than non-superluminescent diodes but for number of applications the benefits of this increased brightness are outweighed by the disadvantage of the much greater temperature sensitivity of lasers and superluminescent diodes. Characteristically a non-superluminescent device exhibits saturation effects in which the rate of increase of light output with increasing drive current begins to fall off at high current drive, whereas in the case of a superluminescent device the characteristic is one of ever-increasing rate of increase of light output with increasing current drive up at least until the lasing threshold is reached.
This invention is specifically directed to ELED's which over a normal operating current range of 0 to 100 mA are substantially non-superluminescent at least down to a temperature of 10.degree. and preferably over the temperature range from 0.degree. to 70.degree. C. Devices which show no signs of superluminescence over this temperature range may begin to show signs of superluminescence at much lower temperatures as a result primarily of the reduction in non-radiactive recombination liable to occur at temperatures significantly beneath 0.degree. C., but for many applications such temperatures lie far enough beneath typical service temperatures, and hence such devices will exhibit substantially no superluminescence in the course of normal use. Furthermore, even at much lower temperatures for example below -30.degree. C. the lasing threshold current still lies outside the normal operating current range of 0 to 100 mA.
For many applications a stripe contact double heterosctucture ELED provides an acceptable source for launching short coherence length (non-laser) light into conventional multimode fibre. In order to raise the lasing threshold of the device, and hence be able to launch more short coherence length light into the fibre, the stripe contact may be arranged to terminate short of the rear end of diode. By terminating the stripe contact well short of one end of the semiconductor chip there is left an unpumped region of the chip beyond the end of the stripe. This unpumped region will remain optically obsorbing and hence minimise optical feedback that would otherwise tend to promote laser action. Examples of such ELED's with full length stripe contacts and with truncated stripe contacts are described for instance in United Kingdom Patent Specification No. 1,600,965.
In a conventional stripe contact ELED the semiconductor lasers are planar, and their composition is chosen to provide a refractive index profile that affords a real waveguiding effect in the direction normal to those layers. However, planar layers produce no lateral waveguiding effect, that is no waveguiding effect in the plane of the layers. The injected carriers under the stripe have the effect of reducing the effective refractive of the material and hence when the device is driven the resulting index profile in the transverse direction (lying in the plane of the layers at right angles to the stripe axis) tends to be anti-waveguiding.
A paper by D. Marcuse and I. P. Kaminow entitled "Computer Model of a Superluminescent LED with Lateral Confinement", IEEE Journal of Quantum Electronics, Volume QE-17, No. 7, July 1981, pages 1234 to 1244, provides a theoretical analysis of the performance of a superluminescent ELED which retains the unpumped region but replaces the truncated stripe contact structure with a more complicated structure that does provide lateral waveguiding. This ELED, which is also described in United Kingdom Patent Specification No. 2090056A, is expressly designed for matching with multimode fibre. The authors of this paper conclude from their analysis that such a structure has at least one major drawback, namely that `it allows a strong reflected wave to build up, causing far more power to be dumped uselessly into the lossy internal diode structure (causing excess heating) than is available at the output end`. The authors then assert that `the reflected wave is particularly harmful because, due to its high energy, it causes gain saturation, thus reducing the growth of the forward traveling wave that delivers power to the fiber`. In conformity with the teaching of this theoretical paper the second author, in conjunction with others, describes in a paper entitled "Lateral Confinement InGaAsP Superluminescent Diode at 1.3 .mu.m", IEEE Journal of Quantum Electronics, Volume QE 19, No. 1, January 1983, pages 78 to 81, (I. P. Kaminow et al) the manufacture of an ELED for coupling to multimode fibre which, instead of having an absorbing region at the rear of the device, has its rear facet provided with a high reflectivity mirror.